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version 1.1.1.2, 2012/02/21 23:50:25	version 1.1.1.5, 2014/06/15 19:46:04
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.TH PCREPARTIAL 3	.TH PCREPARTIAL 3 "02 July 2013" "PCRE 8.34"
.SH NAME	.SH NAME
PCRE - Perl-compatible regular expressions	PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"	.SH "PARTIAL MATCHING IN PCRE"
Line 32 or not a partial match is preferred to an alternative	Line 32 or not a partial match is preferred to an alternative
the details differ between the two types of matching function. If both options	the details differ between the two types of matching function. If both options
are set, PCRE_PARTIAL_HARD takes precedence.	are set, PCRE_PARTIAL_HARD takes precedence.
.P	.P
Setting a partial matching option disables the use of any just-in-time code	If you want to use partial matching with just-in-time optimized code, you must
that was set up by studying the compiled pattern with the	call \fBpcre_study()\fP, \fBpcre16_study()\fP or \fBpcre32_study()\fP with one
PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard	or both of these options:
	.sp
	PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
	PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
	.sp
	PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
	matches on the same pattern. If the appropriate JIT study mode has not been set
	for a match, the interpretive matching code is used.
	.P
	Setting a partial matching option disables two of PCRE's standard
optimizations. PCRE remembers the last literal data unit in a pattern, and	optimizations. PCRE remembers the last literal data unit in a pattern, and
abandons matching immediately if it is not present in the subject string. This	abandons matching immediately if it is not present in the subject string. This
optimization cannot be used for a subject string that might match only	optimization cannot be used for a subject string that might match only
Line 43 matching string, and does not bother to run the matchi	Line 52 matching string, and does not bother to run the matchi
strings. This optimization is also disabled for partial matching.	strings. This optimization is also disabled for partial matching.
.	.
.	.
.SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()"	.SH "PARTIAL MATCHING USING pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
A partial match occurs during a call to \fBpcre_exec()\fP or	A partial match occurs during a call to \fBpcre_exec()\fP or
\fBpcre16_exec()\fP when the end of the subject string is reached successfully,	\fBpcre[16\|32]_exec()\fP when the end of the subject string is reached
but matching cannot continue because more characters are needed. However, at	successfully, but matching cannot continue because more characters are needed.
least one character in the subject must have been inspected. This character	However, at least one character in the subject must have been inspected. This
need not form part of the final matched string; lookbehind assertions and the	character need not form part of the final matched string; lookbehind assertions
\eK escape sequence provide ways of inspecting characters before the start of a	and the \eK escape sequence provide ways of inspecting characters before the
matched substring. The requirement for inspecting at least one character exists	start of a matched substring. The requirement for inspecting at least one
because an empty string can always be matched; without such a restriction there	character exists because an empty string can always be matched; without such a
would always be a partial match of an empty string at the end of the subject.	restriction there would always be a partial match of an empty string at the end
	of the subject.
.P	.P
If there are at least two slots in the offsets vector when a partial match is	If there are at least two slots in the offsets vector when a partial match is
returned, the first slot is set to the offset of the earliest character that	returned, the first slot is set to the offset of the earliest character that
was inspected. For convenience, the second offset points to the end of the	was inspected. For convenience, the second offset points to the end of the
subject so that a substring can easily be identified.	subject so that a substring can easily be identified. If there are at least
	three slots in the offsets vector, the third slot is set to the offset of the
	character where matching started.
.P	.P
For the majority of patterns, the first offset identifies the start of the	For the majority of patterns, the contents of the first and third slots will be
partially matched string. However, for patterns that contain lookbehind	the same. However, for patterns that contain lookbehind assertions, or begin
assertions, or \eK, or begin with \eb or \eB, earlier characters have been	with \eb or \eB, characters before the one where matching started may have been
inspected while carrying out the match. For example:	inspected while carrying out the match. For example, consider this pattern:
.sp	.sp
/(?<=abc)123/	/(?<=abc)123/
.sp	.sp
This pattern matches "123", but only if it is preceded by "abc". If the subject	This pattern matches "123", but only if it is preceded by "abc". If the subject
string is "xyzabc12", the offsets after a partial match are for the substring	string is "xyzabc12", the first two offsets after a partial match are for the
"abc12", because all these characters are needed if another match is tried	substring "abc12", because all these characters were inspected. However, the
with extra characters added to the subject.	third offset is set to 6, because that is the offset where matching began.
.P	.P
What happens when a partial match is identified depends on which of the two	What happens when a partial match is identified depends on which of the two
partial matching options are set.	partial matching options are set.
.	.
.	.
.SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()"	.SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP	If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP
identifies a partial match, the partial match is remembered, but matching	identifies a partial match, the partial match is remembered, but matching
continues as normal, and other alternatives in the pattern are tried. If no	continues as normal, and other alternatives in the pattern are tried. If no
complete match can be found, PCRE_ERROR_PARTIAL is returned instead of	complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
Line 105 example, there are two partial matches, because "dog"	Line 117 example, there are two partial matches, because "dog"
matches the second alternative.)	matches the second alternative.)
.	.
.	.
.SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()"	.SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP,	If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP,
PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without	PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
continuing to search for possible complete matches. This option is "hard"	continuing to search for possible complete matches. This option is "hard"
because it prefers an earlier partial match over a later complete match. For	because it prefers an earlier partial match over a later complete match. For
Line 153 The second pattern will never match "dogsbody", becaus	Line 165 The second pattern will never match "dogsbody", becaus
shorter match first.	shorter match first.
.	.
.	.
.SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()"	.SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16\|32]_dfa_exec()"
.rs	.rs
.sp	.sp
The DFA functions move along the subject string character by character, without	The DFA functions move along the subject string character by character, without
Line 245 If the escape sequence \eP is present more than once i	Line 257 If the escape sequence \eP is present more than once i
line, the PCRE_PARTIAL_HARD option is set for the match.	line, the PCRE_PARTIAL_HARD option is set for the match.
.	.
.	.
.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()"	.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16\|32]_dfa_exec()"
.rs	.rs
.sp	.sp
When a partial match has been found using a DFA matching function, it is	When a partial match has been found using a DFA matching function, it is
Line 268 Notice that when the match is complete, only the last	Line 280 Notice that when the match is complete, only the last
not retain the previously partially-matched string. It is up to the calling	not retain the previously partially-matched string. It is up to the calling
program to do that if it needs to.	program to do that if it needs to.
.P	.P
	That means that, for an unanchored pattern, if a continued match fails, it is
	not possible to try again at a new starting point. All this facility is capable
	of doing is continuing with the previous match attempt. In the previous
	example, if the second set of data is "ug23" the result is no match, even
	though there would be a match for "aug23" if the entire string were given at
	once. Depending on the application, this may or may not be what you want.
	The only way to allow for starting again at the next character is to retain the
	matched part of the subject and try a new complete match.
	.P
You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with	You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
PCRE_DFA_RESTART to continue partial matching over multiple segments. This	PCRE_DFA_RESTART to continue partial matching over multiple segments. This
facility can be used to pass very long subject strings to the DFA matching	facility can be used to pass very long subject strings to the DFA matching
functions.	functions.
.	.
.	.
.SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()"	.SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
From release 8.00, the standard matching functions can also be used to do	From release 8.00, the standard matching functions can also be used to do
Line 293 treat the end of a segment as the end of the subject w	Line 314 treat the end of a segment as the end of the subject w
.sp	.sp
At this stage, an application could discard the text preceding "23ja", add on	At this stage, an application could discard the text preceding "23ja", add on
text from the next segment, and call the matching function again. Unlike the	text from the next segment, and call the matching function again. Unlike the
DFA matching functions the entire matching string must always be available, and	DFA matching functions, the entire matching string must always be available,
the complete matching process occurs for each call, so more memory and more	and the complete matching process occurs for each call, so more memory and more
processing time is needed.	processing time is needed.
.P	.P
\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts	\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
with \eb or \eB, the string that is returned for a partial match includes	with \eb or \eB, the string that is returned for a partial match includes
characters that precede the partially matched string itself, because these must	characters that precede the start of what would be returned for a complete
be retained when adding on more characters for a subsequent matching attempt.	match, because it contains all the characters that were inspected during the
	partial match.
.	.
.	.
.SH "ISSUES WITH MULTI-SEGMENT MATCHING"	.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
Line 315 beginning of a line. There is also a PCRE_NOTEOL optio	Line 337 beginning of a line. There is also a PCRE_NOTEOL optio
doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which	doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
includes the effect of PCRE_NOTEOL.	includes the effect of PCRE_NOTEOL.
.P	.P
2. Lookbehind assertions at the start of a pattern are catered for in the	2. Lookbehind assertions that have already been obeyed are catered for in the
offsets that are returned for a partial match. However, in theory, a lookbehind	offsets that are returned for a partial match. However a lookbehind assertion
assertion later in the pattern could require even earlier characters to be	later in the pattern could require even earlier characters to be inspected. You
inspected, and it might not have been reached when a partial match occurs. This	can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
is probably an extremely unlikely case; you could guard against it to a certain	\fBpcre_fullinfo()\fP or \fBpcre[16\|32]_fullinfo()\fP functions to obtain the
extent by always including extra characters at the start.	length of the longest lookbehind in the pattern. This length is given in
	characters, not bytes. If you always retain at least that many characters
	before the partially matched string, all should be well. (Of course, near the
	start of the subject, fewer characters may be present; in that case all
	characters should be retained.)
.P	.P
3. Matching a subject string that is split into multiple segments may not	From release 8.33, there is a more accurate way of deciding which characters to
	retain. Instead of subtracting the length of the longest lookbehind from the
	earliest inspected character (\fIoffsets[0]\fP), the match start position
	(\fIoffsets[2]\fP) should be used, and the next match attempt started at the
	\fIoffsets[2]\fP character by setting the \fIstartoffset\fP argument of
	\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP.
	.P
	For example, if the pattern "(?<=123)abc" is partially
	matched against the string "xx123a", the three offset values returned are 2, 6,
	and 5. This indicates that the matching process that gave a partial match
	started at offset 5, but the characters "123a" were all inspected. The maximum
	lookbehind for that pattern is 3, so taking that away from 5 shows that we need
	only keep "123a", and the next match attempt can be started at offset 3 (that
	is, at "a") when further characters have been added. When the match start is
	not the earliest inspected character, \fBpcretest\fP shows it explicitly:
	.sp
	re> "(?<=123)abc"
	data> xx123a\eP\eP
	Partial match at offset 5: 123a
	.P
	3. Because a partial match must always contain at least one character, what
	might be considered a partial match of an empty string actually gives a "no
	match" result. For example:
	.sp
	re> /c(?<=abc)x/
	data> ab\eP
	No match
	.sp
	If the next segment begins "cx", a match should be found, but this will only
	happen if characters from the previous segment are retained. For this reason, a
	"no match" result should be interpreted as "partial match of an empty string"
	when the pattern contains lookbehinds.
	.P
	4. Matching a subject string that is split into multiple segments may not
always produce exactly the same result as matching over one single long string,	always produce exactly the same result as matching over one single long string,
especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and	especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
Word Boundaries" above describes an issue that arises if the pattern ends with	Word Boundaries" above describes an issue that arises if the pattern ends with
Line 363 multi-segment data. The example above then behaves dif	Line 422 multi-segment data. The example above then behaves dif
data> gsb\eR\eP\eP\eD	data> gsb\eR\eP\eP\eD
Partial match: gsb	Partial match: gsb
.sp	.sp
4. Patterns that contain alternatives at the top level which do not all start	5. Patterns that contain alternatives at the top level which do not all start
with the same pattern item may not work as expected when PCRE_DFA_RESTART is	with the same pattern item may not work as expected when PCRE_DFA_RESTART is
used. For example, consider this pattern:	used. For example, consider this pattern:
.sp	.sp
Line 412 Cambridge CB2 3QH, England.	Line 471 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 21 January 2012	Last updated: 02 July 2013
Copyright (c) 1997-2012 University of Cambridge.	Copyright (c) 1997-2013 University of Cambridge.
.fi	.fi

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